Heating units of this type can be used, for example, in beverage making machines, such as fully automatic coffee machines having an appliance for heating and frothing milk, in order to heat and froth milk to prepare coffee, milk beverages and frothed milk beverages. In fully automatic coffee machines, it is possible to prepare, for example, milk beverages or mixed milk beverages, such as cocoa, milk coffee or cappuccino, with the heated and frothed milk.
It is known from the prior art that there are appliances for heating and frothing milk which are equipped with a Venturi system, such as described, for example, in EP 0 234 236 B1 and EP 0 344 859 B1. In these appliances, hot steam is used both as a heating medium and as a propellant for, respectively, heating and propelling the milk. However, the heat output of appliances equipped with a Venturi system is low, and the product properties, such as the temperature of the heated milk or the porosity of the milk foam, can be controlled only to a limited extent.
An improved appliance for heating and/or frothing milk has been disclosed in WO 2013/064232 A1. This appliance comprises a heating unit for heating the milk with steam, with hot steam acting as a heat carrier in the heating unit operating as an injector being injected into the milk. The heating unit comprises an inner flow channel for the hot steam and a flow channel for the milk and/or the milk foam, which latter channel coaxially surrounds the inner flow channel, with the two flow channels being connected to one another by means of a plurality of radial channels that extend at right angles to the direction of flow. This heating unit allows the milk and the hot steam to be supplied separately, which makes it possible to froth the milk to produce a milk foam independently of heating. As a result, the properties and the composition of the milk foam can be controlled independently of the heating process. In addition, heating the milk based on the principle of steam injection also has the advantage that the heating process does not impair the quality of the milk foam since, due to the separately supplied steam, it can be ensured that the temperature of the heated milk is accurately controlled and that the thermal inertia of the preparation system is minimized.
It has been found, however, that in the heating unit disclosed in WO 2013/064232 A1, milk-stone and/or milk incrustations and lime scale can form in the narrow radial channels which connect the inner flow channel for the steam with the outer flow channel for the milk. Such deposits and incrustations in the radial channels can form during or after the heating phase when the milk is heated to high temperatures and cause the radial channels to become narrower, which in turn reduces the heat output and the efficiency of the heating unit. Removing the deposits and incrustations from the radial channels is extremely labor-intensive and time-consuming.